1. Field of the Invention
The present invention pertains to a method for processing a video signal and, more particularly, to adaptively sensing certain video signal content in specific areas of a video signal and controlling the presence of selected copy protection signals in response to the certain video signal content.
2. Description of the Prior Art
The present invention relates to a method and apparatus for improving playability of certain copy protection techniques that modify a color video signal in such a manner that a conventional television receiver produces a normal color picture from the modified signal, whereas a videotape recording made from the modified signal exhibits very annoying color interference.
Video anticopy processes are well known. An example is U.S. Pat. No. 4,631,603 (xe2x80x2603) issued Dec. 23, 1986 to J. Ryan, and incorporated herein by reference. The xe2x80x2603 patent discloses inter alia (see Abstract):
xe2x80x9cA video signal is modified so that a television receiver will still provide a normal color picture from the modified video signal while the video tape recording of the modified video signal produces generally unacceptable pictures. The invention relies on the fact that typical video cassette recorder automatic gain control systems cannot distinguish between the normal sync pulses (including equalizing or broad pulses) of a conventional video signal and added pseudo-sync pulses. Pseudo-sync pulses are defined here as any other pulses which extend down to normal sync tip level and which have a duration of at least 0.5 microseconds. A plurality of such pseudo-sync pulses is added to the conventional video signal during the vertical blanking interval, and each of said pseudo-sync pulses is followed by a positive pulse of suitable amplitude and duration. As a result, the automatic gain control system in a videotape recorder will make a false measurement of video level which causes an improper recording of the video signal. The result is unacceptable picture quality during playback.xe2x80x9d
The xe2x80x2603 patent, column 2, beginning at line 5, states that the added pulse pairs (each pair being a negative-going pseudo-sync pulse followed by a positive-going xe2x80x9cAGCxe2x80x9d pulse) cause an automatic level (gain) control circuit in a videotape recorder to erroneously sense video signal level and produce a gain correction that results in an unacceptable videotape recording.
Another example of a video anticopy process is disclosed in U.S. Pat. No. 4,819,098 (xe2x80x2098) issued on Apr. 4, 1989 to J. Ryan, and incorporated herein by reference. The xe2x80x2098 patent discloses (see Abstract):
xe2x80x9cA video signal is modified so that a television/monitor receiver still produces a normal picture from a modified signal, whereas a videotape recording of this signal produces generally unacceptable pictures. Videotape recorders have an automatic gain control circuit which measures the sync pulse level in a video signal and develops a gain correction signal for keeping the video signal level applied to an FM modulator in the video tape recording system at a fixed, predetermined value. A plurality of pulses are added immediately following a respective trailing edge of a normally occurring sync pulse. These added pulses are clustered at the vertical blanking interval of each field to minimize the affect of the same on the viewability of the picture defined by the signal while causing the automatic gain control circuit in a recorder to assess the video level at many times its actual value. The sync pulses themselves can also be at a reduced level, in order to enhance the effectiveness of the process.xe2x80x9d
Therefore these prior art xe2x80x9cbasic anticopy processesxe2x80x9d cause an abnormally low amplitude video signal to be recorded when a copy is attempted. Some of the effects observed when the illegal copy is replayed are horizontal tearing (positional displacement) and vertical displacement of the picture. Whether this occurs or not is often large dependent on the picture content, i.e., presence of white (light) and black (dark) areas in the picture. Therefore this prior art process, while generally providing excellent copy protection and normal picture quality in most types of television sets, may display a minor amount of hooking or tearing at the top portion of the picture in a very small minority of television sets. Thus there is a need for an adaptive control of the copy protection process which removes the visibility of this minor hooking or tearing of the displayed picture when viewing the copy protection signal.
It is an object of the present invention to provide an improved method and apparatus of modifying a video signal so that all conventional television receivers produce a normal color picture from the modified signal, whereas a videotape recording made from the modified signal exhibits annoying interference, thus discouraging or inhibiting videotaping of the signal.
It is a further object of the invention to provide a method and apparatus for detecting the scene content in an upper portion of a video picture, and for adaptively activating or deactivating the insertion of a copy protection signal depending upon preselected conditions which are detected in the scene.
To this end, the invention includes a circuit in a conventional video processing system of, for example, a digital video disk or tape master production facility, which circuit detects picture information, which would be noticeably altered by hooking, in the upper portion of a television picture. Upon detecting a preselected amount of the picture information which would be noticeably altered by hooking, the circuit supplies a signal, for example, a digital bit, which disables the copy protection process so that the copy protection signal is not added to the video signal in the upper portion of the video picture. In the absence of the noticeably alterable picture information in the upper portion of the picture, the copy protection process is enabled. Likewise, the copy protection process is enabled for the remaining majority of the video picture in the usual procedure for applying copy protection.
These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims and accompanying drawings.